Efficient use of the light source, in most cases an arc lamp is important for cost efficient implementation of high brightness projectors. The use of ever smaller micro-displays creates a demand for light sources with a lower etendue, or in the case of an arc lamp a smaller arc gap. Especially for higher power lamps such a smaller arc gap cannot be achieved unless at the cost of a significant reduction of the lamp life. When combining multiple light sources to increase the brightness the etendue of the light sources add up so that efficiency drops and the effective brightness increase from adding a light source is less then the expected multiple of a single light source output.
More particularly, the present invention is in the technical field of DLP™ projection technology, such as e.g. based on the Texas Instruments digital micro-mirror device DMD™. This DMD consist of a 2-dimensional array of microscopic mirrors that can tilt to two positions typically plus or minus 10 to 12 degrees from an undriven flat position. This way each pixel in the chip can direct the light that hits it in either the ‘on state’ or the ‘off state’. The on-state light reaches the screen through the projection lens and gives a bright pixel, while the off state light is absorbed inside the projector; this results in a dark pixel. Fast transitions between the two states in a pulse width modulation scheme are used to create a pixel with desired intensity level.
High-end applications use 3 DMD's: one for the red, green and blue light. Colours are split and recombined by using colour prism architecture. This configuration offers the best light-output and saturated colours.
Low-end applications use a single DMD that sequentially generates the different colour components. The sequence of colours from a white light source is usually obtained by means of a rotating colour wheel. This colour wheel will pass one spectral primary band from the white light source while reflecting the other spectral components. Typically red, green and blue spectral bands are passed sequentially. Since ⅔ of the light from the light source is rejected the efficiency of a single-DMD system is much lower then the efficiency of a 3-DMD system. In some applications a white segment is added to the colour wheel to increase the brightness in white. However the brightness of the pure primary (red, green and blue) colours and pure secondary (yellow, cyan and magenta) colours obtained thereby remains unchanged, so that adding a white segment results in a loss of colour saturation.
In both of the above architectures a total internal reflection or TIR prism is used to reflect the incoming light from the light source to the DMD while passing the light that bounces off the DMD.
The prior art in such technical field includes publications that suggest the use of pulsed LED light sources to increase the lumens output of a projection device; such as the article “increased lumens per etendue by combining pulsed LED's” published in the proceedings of SPIE Vol. 5740. The suggested solutions however require the use of additional moving elements or switchable retarders, both difficult to implement especially with large and high power lamp light sources. Also the implementation suggested with the switchable retarders relies on the polarization of the light source, a solution that is only efficient when the display device operation is based on the use of polarized light, which is not the case for DLP™ projection technology.